1. Field of the Invention
The invention pertains to an imaging system comprising means for compensating vignetting in an image supplied by an image generation means. The invention also ,elates to an x-ray examination apparatus comprising such imaging system.
An x-ray examination apparatus comprising an imaging system of said kind has been described in the European Patent Application EP 0 333 276 which corresponds to commonly owned U.S. Pat. No. 4,969,176.
Loss of brightness towards a periphery of an image (vignetting) is particularly likely to occur in a lens system containing beam-stops or tandem-sets of lenses. In medical x-ray radiography various further origins of vignetting are known, e.g. variations in intensity in an x-ray beam emitted by an x-ray source, the geometry of the x-ray detection screen, e.g. an input screen of an x-ray image intensifier or the substantially cylindrical shape of a patient to be examined. Vignetting is an image perturbation causing disturbing artifacts or which in medical x-ray images impairs diagnostic quality of an x-ray image. In the x-ray examination apparatus as described in the cited European Patent Application, vignetting is avoided by employing a scatter radiation grid having a transitivity which increases towards the periphery of the x-ray image. The transitivity profile of said scattered-radiation grid is fixed, so that optimum suppression of vignetting is achieved only for a particular set of conditions for generating an image.